Direct abstraction

A free-radical process in which the carbene directly abstracts a hydrogen from the substrate to generate a pair of free radicals ... 

This observation is the first part of the cancellation puzzle [20, 21, 27, 29]. We know from Section lll.B that we should be able to solve it directly by applying Eq. (19), which will separate out the contributions to the DCS made by the 1-TS and 2-TS reaction paths. That this is true is shown by Fig. 9(b). It is apparent that the main backward concentration of the scattering comes entirely from the 1-TS paths. This is not a surprise, since, by definition, the direct abstraction mechanism mentioned only involves one TS. What is perhaps surprising is that the small lumps in the forward direction, which might have been mistaken for numerical noise, are in fact the products of the 2-TS paths. Since the 1-TS and 2-TS paths scatter their products into completely different regions of space, there is no interference between the amplitudes f (0) and hence no GP effects. 

Due to the high rate of reaction observed by Meissner and coworkers it is unlikely that the reaction of OH with DMSO is a direct abstraction of a hydrogen atom. Gilbert and colleagues proposed a sequence of four reactions (equations 20-23) to explain the formation of both CH3 and CH3S02 radicals in the reaction of OH radicals with aqueous DMSO. The reaction mechanism started with addition of OH radical to the sulfur atom [they revised the rate constant of Meissner and coworkers to 7 X 10 M s according to a revision in the hexacyanoferrate(II) standard]. The S atom in sulfoxides is known to be at the center of a pyramidal structure with the free electron pair pointing toward one of the corners which provides an easy access for the electrophilic OH radical. 

An extensive review of the literature reveals that the only studies of vibrational effects in insertion chemistry have focused on reactions of 0(1D)175-177 and C(1D)177,178 with H2. Since there is no potential energy barrier to insertion in these systems, reaction proceeds readily even for unexcited reactants.179 Since the efficiency of vibrational excitation was 20% in both studies, due to the large cross-sections for ground state reactions, only small changes were observed in the experimental signal. From an analysis of the product distributions, it was concluded that while H2(v = 0) primarily reacted via an insertion mechanism, direct abstraction seemed to become important for = 1). For 0(1D), this is similar to behavior at elevated collision energies.180... 

With increasing pressure, the fraction of the activated complex that is stabilized will approach unity [17], As the temperature increases, the route to the olefin becomes favored. The direct abstraction leading to the olefin reaction (3.47) must therefore become important at some temperature higher than lOOOK [17a],... 

In parallel with the direct abstraction mechanism, a second important mechanism of epimerization can occur via an intermediate that can be produced from any type of activation. This common intermediate is the oxazol-5(4//)-one, produced by nucleophilic attack of the penultimate carbonyl oxygen atom attached to the amino group on the activated carboxy carbon atom (Scheme 4). 

The UNCAs are ideal activated intermediates for the study of intrinsic rates of racemization. Epimerization can occur only via the direct abstraction mechanism, because the five-membered ring structure precludes formation of an oxazol-5(4//)-one. In addition, the UNCA maintains its structural integrity during the epimerization process. A report by Rom off129 describes two simple methods for the measurement of intrinsic rates of racemization under a wide variety of reaction conditions utilizing UNCAs as the prototypical activated intermediates. 

In short, the OH + HN03 reaction likely proceeds in part via a direct abstraction and in part via complex formation, with decomposition of the complex to H20 + NOv There is no experimental evidence for the existence of a minor channel producing H202 + N02, although a small contribution at lower temperatures cannot be ruled out. 

Data on alkyl radical oxidation between 300° and 800°K. have been studied to establish which of the many elementary reactions proposed for systems containing alkyl radicals and oxygen remain valid when considered in a broad framework, and the rate constants of the most likely major reactions have been estimated. It now seems that olefin formation in autocatalytic oxidations at about 600°K. occurs largely by decomposition of peroxy radicals rather than by direct abstraction of H from an alkyl radical by oxygen. This unimolecular decomposition apparently competes with H abstraction by peroxy radicals and mutual reaction of peroxy radicals. The position regarding other peroxy radical isomerization and decomposition reactions remains obscured by the uncertain effects of reaction vessel surface in oxidations of higher alkanes at 500°-600°K. 

A remaining critical mechanistic question deals with the mode of product formation from the ion pair formed upon H abstraction. At this point, the reaction can be consummated either by transfer of H- from B to the carbonyl carbon (path a, Scheme 30), or direct abstraction of H- from another R3SiH reagent (path b).248 In this latter scenario, the borane becomes a spectator in the reaction, and the true catalyst is the [R3Si]+ cation. To probe this question, we performed the experiment depicted in Scheme 31. In the case of path b, both pairs of isotopomers should be observed, while if path a is operative, only the unscrambled products should be present. In fact, the product mixture consistent with... 

The second channel for the reaction exists that leads to the same products. This is the direct abstraction of an oxygen atom from N20 by a silyl radical. Figure 7.2f shows the structure of the transition state (TS-4) corresponding to this channel. According to the calculations, this channel is characterized by high activation energy (10.6kcal/mol). 

Especially for alkyl halides 6 the transfer of a single electron from the metal center is facile and occurs at the halide via transition state 6C, which stabilizes either by direct abstraction of the halide to a carbon-metal complex radical pair 6D or via a distinct radical anion-metal complex pair 6E. This process was noted early but not exploited until recently (review [45]). Alkyl tosylates or triflates are not easily reduced by SET, and thus Sn2 and/or oxidative addition pathways are common. The generation of cr-radicals from aryl and vinyl halides has been observed, but is rarer due to the energy requirement for their generation. Normally, two-electron oxidative addition prevails. 

Entry no. 2 of Table 16 introduces the most remarkable aspect of Co(III) chemistry, namely its ability to oxidize non-activated C—H bonds (for a recent study, see Jones and Mellor, 1977) and we immediately see that Marcus theory here completely rules out the possibility of an initial electron-transfer step. This is predicted to be ca. 1016 times slower than what is actually observed. On the other hand, the theory correctly predicts the rate constant for oxidation of naphthalene under the same conditions, and the postulated direct abstraction of a 7t-electron is thus feasible (Cooper and Waters, 1967). The Co(III) trifluoroacetate study of entry no. 3, including only substrates without an alkyl side-chain, however, cannot be fitted to any physically realistic set of E°, A parameters. With E° = 1.83 V (value in 1.0 M HC104) A ought to be ca. 40 kcal mol-1 and the slope of the log k/AG0 regression line ca. —0.6 the A value is then in reasonable agreement with the estimated one but not the slope. With E° = 3.2 V (clearly not a physically very realistic standard potential) a A value of ca. 80 kcal mol-1 is required. This seems to be far too large for such a system. 

Thus, it can be suggested that DPPH is unable to abstract hydrogen from a CH3 group and that in the mechanism assumed by Proll and Sutcliffe (1963) not only DPPH but a more reactive radical species has to be involved. This means a modification of the direct abstraction mechanism. A possible change in the mechanism of DPPH reactions could also be conjectured from the work of Hogg et al. (1961) who presented a very sharp break in the Hammett plot correlating the... 

The rebound mechanism, though in a modified version, has been recently supported by theoretical calculations of KIF using the density functional theory (Yoshizawa et al., 2000). The calculations demonstrate that the transition state for the H-atom abstraction from ethane involves a linear [FeO.H...C] array a resultant radical species with a spin density of nearly one is bound to an iron-hydroxy complex, followed by recombination and release of product ethanol. According to the calculation of the reaction energy profile, the carbon radical species is not a stable reaction intermediate with a finite lifetime. The calculated KIF at 300 K is in the range of 7-13 in accord with experimental data and is predicted to be significantly dependent on temperature and substituents. It was also shown from femtosecond dynamic calculations in the FeOVCH4 system that the direct abstraction mechanism can occur in 100-200 fs. 

In a similar manner, sensitizer can diffuse into the alcohol-preswollen cellulose and either directly abstract hydrogen atoms (Equations 1 and 4) or rupture bonds as shown in Equation 5 to form additional copolymerization sites. 

The calculated rate coefficients at 298 K are 1.10 and 1.45 x 10 L/(cm s) for formaldehyde and acetaldehyde, respectively [27]. These values are in excellent agreement with the experimental data. The structure of the transition states are shown in Figure 12.4. Both are early transition states consistent with reactions with very low barriers. The transition state of OH hydrogen abstraction from acetaldehyde is earlier than the one of formaldehyde. Additionally, the possibility of the addition channel was excluded due to the large barrier associated to this process. The temperature dependence of the rate coefficient was not studied. In a more recent work [105] the level of the calculations was increased, the temperature dependence of the rate coefficient and the role of direct abstraction were studied. All the main conclusions from these articles are now accepted in recent works [106-108], and it is well known that the hydrogen abstraction is the main reaction channel if not unique [108,109]. 

Figure 12.6 shows the Arrhenius plots for the different reaction channels contributing to propanone + OH overall reaction. None of the rate constants of the individual paths matches the experimental ones in the studied temperature range. Only at the highest temperature (440 K) the contribution of the direct abstraction is larger than that of the complex abstraction. At the lowest temperatures the direct abstraction is negligible while at intermediate temperatures both are important. This is in excellent agreement with the proposal of two channels one complex and one direct H abstraction. 

Let us insist on the nature of the path used to calculate the previous rate coefficients. Direct abstractions refer to elementary reactions with the reactants are converted into abstraction products without any intermediate step. On the other hand, in the complex path a weakly bonded complex is formed in the entrance channel. For H abstraction from alpha sites in ketones it is important to distinguish between eclipsed and alternated hydrogens [122]. The transition state of the abstractions from eclipsed alpha sites are the only one directly connected to the reactant complex. 

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